共Ga,Mn兲As 共Ref.

(1)

Identifying the character of ferromagnetic Mn in epitaxial Fe/(Ga,Mn)As heterostructures

M. Sperl,¹F. Maccherozzi,²F. Borgatti,³A. Verna,⁴G. Rossi,^4,5M. Soda,¹D. Schuh,¹G. Bayreuther,¹W. Wegscheider,¹ J. C. Cezar,⁶F. Yakhou,⁶N. B. Brookes,⁶C. H. Back,¹and G. Panaccione^4,

*

1Institut für Experimentelle Physik, Universität Regensburg, D-93040 Regensburg, Germany

2Soleil Synchrotron, L’Orme des Merisiers Saint-Aubin, BP 48, F-91192 Gif-sur-Yvette, France

3ISMN-CNR, via Gobetti 101, Bologna, Italy

4Laboratorio Nazionale TASC, INFM-CNR, in Area Science Park, S.S. 14, Km 163.5, I-34012 Trieste, Italy

5Dipartimento di Fisica, Università di Modena e Reggio Emilia, Via A. Campi 231/A, I-41100 Modena, Italy

6European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble, France

共Received 18 November 2009; revised manuscript received 22 December 2009; published 27 January 2010

兲

We demonstrate that the growth of Fe/共Ga,Mn兲As heterointerfaces can be efficiently controlled by epitaxy and that robust ferromagnetism of the interfacial Mn atoms is induced at room temperature by the proximity effect. X-ray magnetic circular dichroism and x-ray resonant reflectivity data, supported by theoretical calculations, were used to monitor both temperature and magnetic field dependence of the Mn magnetic moment in the semiconducting host. We identify distinct Mn populations, each of them with specific magnetic character.

DOI:10.1103/PhysRevB.81.035211 PACS number共s兲: 75.50.Pp, 71.20.Nr, 78.70.Dm

I. INTRODUCTION

Diluted magnetic semiconductors

共DMS兲

hold promises for integrating spin control in electronic devices.^1,2Although the correlation between magnetic and transport properties in DMS is known to be a crucial ingredient toward possible applications, the physical mechanisms governing both the magnetic character and the magnetization alignment of the metallic dopants in the semiconducting environment are open challenges of present DMS research.^3–5One further key subject arises from the Curie temperature, presently around 200 K in the most representative DMS material

共Ga,Mn兲As 共Ref.

6兲 while ferromagnetic

共FM兲

behavior well beyond room temperature

共RT兲

would be required in future spintron- ics devices. A promising direction that goes beyond conven- tional methodologies is to tailor novel properties by exploit- ing interface effects in highly controlled heterostructures

共HS兲,

as extensively demonstrated in oxide-based materials.^7–9 Following this approach, recent experimental efforts aimed at tuning specific magnetic properties in FM metal/DMS-based interfaces. In the case of direct exchange- coupled HS, the investigation of spin valve effect in MnAs/

共Ga,Mn兲As at low temperature 共4.2 K兲

found the apparent formation of a inhomogeneous magnetic spring in the

共

Ga,Mn

兲

As region¹⁰ and a decrease in exchange coupling after insertion of a spacer layer

关MnAs/

p-GaAs/共Ga, Mn兲As兴.¹¹Differently, HS with 3dFM metals revealed

共i兲

independent magnetization switching in

共Ni

80Fe₂₀

兲

/

共Ga, Mn兲As, despite direct contact

¹² and

共ii兲

FM behavior of Mn at RT at nonepitaxial Fe/

共

Ga,Mn

兲

As interfaces, with antiparallel alignment of the Fe and Mn moments.¹³ A firm understanding of the mechanisms in- volved in these effects requires on one side a full control of HS growth and on the other side the ability to probe the electronic and magnetic properties in a chemical and depth sensitive way. Here we report polarization-dependent x-ray experiments on fully epitaxial Fe/共Ga,Mn兲As HS, where we are able to follow the field, temperature, and depth evolution of the different magnetic configurations of the Mn ions. Our

results reveal the presence of a electronic configuration of the Mn ions under the proximity effect of the Fe overlayers, indicating that

共a兲

Mn hybridization is playing a fundamental role in the magnetic properties of the system and

共b兲

paramagnetic and ferromagnetic Mn

共

antiparallel to Fe

兲

have distinct spectroscopic fingerprints in an applied magnetic field, i.e., the different magnetic behavior in a sizeable part of the Mn atoms at the interface

共ⲏ1 nm兲

is “switched on” by the Fe overlayer.

II. EXPERIMENTAL DETAILS

The ferromagnetic

共Ga,Mn兲As samples 共Mn 3 – 8 % dop-

ing, 50–150 nm thick兲have been grown by molecular-beam epitaxy

共MBE兲. The rate for the low-temperature growth was

about 0.6 Å/s

共

230° – 250 ° C

兲

. Subsequently the samples were transferred to a metal MBE for Fe evaporation. An optimized protocol to obtain contamination free and ordered surfaces was used, by combining low-temperature ultrahigh vacuum annealing

共150 ° C兲

and soft Ar⁺sputtering

共500 eV,

45° incidence angle

兲

for a typical time of 30 min. Details are given elsewhere.^14,15 Reference samples were obtained by HCl etching. The Fe growth rate was controlled in situ by quartz monitors and the Fe overlayers were prepared as stepped wedges with discrete thickness values to ensure that the growth conditions were identical for all thicknesses in the covered range from 0 to 23 ML. The resulting well ordered surfaces were verified by reflective high-energy electron dif- fraction. Samples were covered by a 8-nm-thick epitaxial Au

共

001

兲

layer to prevent oxidation. The magnetic anisotropy axis of both Fe and

共Ga,Mn兲As layer, determined by a super-

conducting quantum interference device magnetometer and by magneto-optical Kerr effect

共MOKE兲, closely resembles

Fe/GaAs共001兲and the nontreated

共Ga,Mn兲As/GaAs共001兲, re-

spectively, excluding sizeable variations in the magnetic properties after the Ar⁺ bombardment.^14–16 Moreover, the Curie temperature of the sputtered

共Ga,Mn兲As is identical to

the one of the as-grown samples

共T

c= 67⫾3 K兲. Figure1共a兲 presents MOKE results. At the wavelength used the Kerr

(2)

contrast for

共Ga,Mn兲As and Fe has opposite sign which

makes it easy to separate the switching behavior of the two different layers. We show data at 300 K where only the Fe film produces a magnetic contrast and at 6.5 K for the Fe/

GaMnAs film and for a pure

共Ga,Mn兲As film for compari-

son. We acquired x-ray absorption spectroscopy

共XAS兲

and x-ray circular magnetic dichroism

共XMCD兲

spectra at the Mn and Fe L edges at the ID08 dragon beamline

共ESRF,

France

兲

and at the APE beamline of the Elettra Synchrotron

共Trieste, Italy兲

in a temperature range of 10–300 K and a base vacuum ⬍5⫻10⁻¹⁰ mbar. Total electron yield

共TEY兲

and photon yield

共PY, bulk sensitivity

⬎40 nm兲 were acquired simultaneously. All spectra were normalized by the

intensity of the incident beam. The TEY Fe XAS signal has been corrected for saturation effects, whereas PY mode has been preferred for Mn, due to both the need of bulk sensitivity and to the dilution in GaAs, resulting in negligible self-absorption effects of the fluorescence signal. X-ray resonant magnetic scattering

共XRMS兲

measurements have been carried out at ID08 in specular reflection geometry by fixing the incoming photon energy at the L₃Mn absorption peak and scanning the grazing angle␪ from 0° to 40°. Right cir- cularly polarized radiation was used and reflectivity measurements were performed in remanence after having applied the magnetic field in opposite directions. Simulation and fitting of the x-ray resonant reflectivity measurements have

normalizedKerrintensity(arb.units)

-1000 0 1000

H (Oe)

(a)

Fe/(Ga,Mn)As T=6.5 K

Fe/(Ga,Mn)As T=300 K

(Ga,Mn)As T=6.5 K

H_c,Fe=110 Oe

Hc,(Ga,Mn)As=55 Oe

H_c,Fe=207 Oe Hc,(Ga,Mn)As=80 Oe

20 10 0

TEY-XMCD(%)

740 730 720 710 700

Photon energy (eV) 0.6

0.4

0.2

0.0

TEY-XAS(arb.units)

(c) P⁺, M⁺

P⁺, M^-

Fe L_2,3

(b)

(Ga,Mn)As Fe Au

2 nm

0.4

0.2

0.0

XAS(arb.units)

655 650 645 640

Photon energy (eV)

0 -1 -2

PY-XMCD(%)

dE=0.5eV

(d)

PY TEY

Fe_t= 23ML Fe_t= 0ML

Mn L_2,3

FIG. 1. 共Color online兲共a兲Normalized MOKE data measured at 300 and 6.5 K for Fe/共Ga,Mn兲As and 共Ga,Mn兲As. The Kerr contrast 共measured with␭= 632 nm兲is opposite for Fe and共Ga,Mn兲As which makes it possible to easily distinguish the two layers. The coercive field of Fe increases when going to lower temperatures as expected. A slight increase inH_cis observed for the pure共Ga,Mn兲As when Fe is deposited on top.共Panel b兲High-resolution cross-sectional TEM image of the epitaxial Fe共2 nm兲/共Ga,Mn兲As共50 nm兲sample, capped with 8 nm of Au. The interface region is sharp and no intermixing is visible. The black horizontal line共bottom right兲 indicates the scale共2 nm兲. 共Panel c兲 FeL_2,3 edge spectra as measured in TEY mode 共T= 120 K , H= 0.04 T , Fe= 23 ML兲, with circular polarization 共P⁺兲 and opposite magnetization共M⁺,M⁻兲. Black arrows indicate the XAS共left兲and XMCD共right兲scale, respectively.共Panel d兲MnL_2,3edge spectra as measured in TEY and PY mode共T= 120 K , H= 0.04 T , Fe= 23 ML兲. Black arrows indicate the XAS共left兲and XMCD共right兲scale.

Mn XAS spectra acquired in PY mode 共blue/dark gray continuous line兲 and TEY mode 共red/gray points兲 differ by 0.5 eV and display different fine structure atL₃edge. XMCD signal at the MnL₃edge共green/light gray curve兲is observed, indicating antiparallel magnetic coupling of Mn with respect to the Fe. No XMCD signal is measured in absence of the Fe overlayer共yellow/light gray curve in panel d, Fe= 0 ML兲.

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been performed using the Pythonic Programming for Multilayer

共

^PPM

兲

software.^17,18

III. RESULTS AND DISCUSSION

Figure 1 depicts the magnetic and structural characteris- tics of the Fe/共Ga,Mn兲As HS. The crystallinity of the Au/Fe/

共

GaMn

兲

As stack is confirmed by the transmission electron microscopy

共TEM兲

image

共panel b兲, revealing a sharp inter-

face between Fe and

共Ga,Mn兲As. Reference XAS-XMCD

signals from the FeL_2,3and MnL_2,3edges are displayed in panels

共

c

兲

and

共

d

兲

, for 23 ML Fe and 0.04 T at 120 K, i.e., well aboveTcof the

共Ga,Mn兲As substrate. The PY spectrum

is located at

⬵0.5 eV lower binding energy 共BE兲

and it is less structured than the TEY one. The PY-XMCD displays FM behavior and antiparallel

共AP兲

Mn orientation.¹³ We stress that, above T_c, a XMCD signal at MnL_2,3 edge is measured only in presence of the Fe overlayer. The yellow curve in panel

共d兲

shows no XMCD at the Mn edge in the region free of Fe, thus indicating a magnetic behavior switched on by the proximity effect of the Fe film.

From spectroscopic results, it is generally agreed that the two prevailing Mn electronic structures in

共Ga,Mn兲As are

Mn_Ga

共Mn substitutional on the Ga site兲

and Mn forming oxides or aggregates near the surface, with a hybridd⁴-d⁵-d⁶ and a purelyd⁵ electronic configuration

共

localized Mn

兲

, respectively. The latter configuration has a more structured XAS spectrum, located at higher BE with respect to the d⁴-d⁵-d⁶ one.^19–22A recent detailed study on the depth con- centration of Mn in

共Ga,Mn兲As reveals the presence of Mn

withd⁵configuration up to 6 nm from the interface, coexist- ing with Mn_Ga and not related to oxide.²³ Results in Fig.1, obtained on controlled epitaxial systems with negligible oxide contribution indicate a surface-interface region with d⁵-like character

共TEY spectrum兲

and a bulk region, mainly representative of the Mn_Ga

共

PY spectrum

兲

. For the sake of simplicity, we will refer in the following to Mn-1

共Mn

Ga

substitutional兲, Mn-2

共d

⁵-like Mn兲, and to Mn-AP

共FM Mn

antiparallel to Fe兲. The evolution of the MnL_2,3XMCD signal vs applied magnetic field is presented in Fig. 2共a兲

共

T= 120 K, Fe 23 ML

兲

. At low field

共

0.04 T

兲

only the Mn-AP dichroic component is visible at the L₃ edge, confirming previous results.¹³ Increasing the field, further components appear at lower BE, evolving in a structured line shape with a double peak at the L₂ edge and with the magnetization aligned parallel to the field. These components correspond to the paramagnetic contribution of Mn-1. The XAS/XMCD arising from Mn-1 has been associated to a mixed-valence ground state due to the large Mn 3d hybridization with the surrounding 4spstates.^19,24,25Numerical calculations indicated a mixed 80% d⁵-20% d⁶L ground state

共L

is a ligand hole兲.^24,25 In Mn-AP, both the energy shift of the XMCD minimum and the line shape suggest a different valence state with lower occupancy of the 3d levels, imply- ing a lowering of the d⁶ character and increment of thed⁵ one.

In Fig.2共b兲the Mn-AP XMCD at 0.04 T is compared to atomic multiplet calculations, namely, a pure Mnd⁵ground- state configuration based on the ligand field model, for an

isolated ion in spherical symmetry,^24,26,27 and a d⁵

共50%兲-d

⁶

共50%兲

mixed-valence configuration. The d⁵

共50%兲-d

⁶

共50%兲

mixed-valence XMCD has been calculated within the impurity Anderson model with the ad- ditional parameters: E_g

共

d⁶

兲

= −3 eV, E_f

共

d⁶

兲

= −4 eV, and T_t2g= 2Teg= 2. Eg and Ef are related to the charge-transfer energies for ground and excited states whileT_t2gandTegare the charge-transfer integrals for t_2g and eg orbitals, respectively.²⁸ Introducing in the calculation furtherd⁴character ord⁴-d⁵ mixing withoutd⁶terms does not provide any better agreement to the experimental Mn XMCD. The comparison to the calculation highlights the smearing of the multiplet features in the experimental results and reveals unam- biguously that Mn-AP XMCD does not bear a pure atomiclike character, suggesting a mixed-valence state with itinerant character. Moreover, the broadened features at the high-energy side of theL₃XMCD that appear for a metalli- clike condition are totally absent, thus excluding Mn alloying/clustering. The observed mixed-valence features suggest a rearrangement of the local density of states associated to changes in the local Mn environment that may be interpreted by the formation of an impurity band, similar to half-metallic systems, where Mn atoms have localized magnetic moments but delocalized 3d electrons.^29,30

To disentangle the Mn-AP ferromagnetic character from the paramagnetic one, we now need to assign the different contributions to the XMCD spectra. To this aim, a fitting procedure was performed, using a linear combination of ex-

XMCD-PY(arb.units)

655 650 645 640

Photon Energy (eV) (x4)

(a)

Mn-AP Mn-1

5 T 3 T 2 T 1.5 T 0.5 T 0.04T

XMCD(arb.units)

655 650 645 640

Photon Energy (eV) (b) Calc. d⁵-d⁶L

Calc. d⁵

Exp.

Mn L_2,3XMCD

FIG. 2. 共Color online兲共Panel a兲 Evolution of the Mn XMCD line shape vs applied magnetic field 共T= 120 K , Fe= 23 ML兲, measured in PY mode. The inset shows a sketch of the experimental geometry. 共Panel b兲 Comparison of calculated and experimental 共120 K, 0.04 T兲Mn-AP XMCD. All curves have been normalized to the minimum of the experimental XMCD and are arbitrarily shifted in energy to align the theoretical to the experimental result. Features related to multiplet splitting effects are absent or smeared, as indicated by the vertical dashed lines. The interatomic screening and excessive electron-electron repulsion of the free ion were taken into account by reducing the Slater integrals to 80%. The 2pspin-orbit interaction was scaled to 103%. An exchange field of 0.01 eV was applied along the magnetization direction. The calculated spectra include a Lorentzian of 0.25共0.4兲eV for theL₃共L₂兲edge to account for intrinsic linewidth broadening and a Gaussian of␴= 0.1 eV for instrumental broadening. For sake of comparison, all calculated spectra are normalized to the same amplitude and arbitrarily aligned in binding energy.

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perimental Mn-1,2 XAS and XMCD spectra from Ref. 22, constrained to reproduce XAS and XMCD data vs magnetic field, temperature, and Fe thickness. The Mn-AP XMCD cannot be reproduced by the superposition of Mn-1 and Mn-2, and has to be explicitly included in the fitting routine.

Results for 120 K, 1 T, and 23 ML of Fe are presented in Fig.

3共a兲. We find only two contributions in the XMCD spectra:

paramagnetic from bulk Mn-1

共T

⬎Tc

兲

and ferromagnetic from Mn-AP. Although a 1 T field is applied, no paramagnetic contribution associated to Mn-2 character is found, thus confirming the absence of Mn oxide in the epitaxial HS.

Disentangling the different Mn character in XMCD spectra allows tracing the evolution vs magnetic field, i.e., to recon- struct site selective

共Mn-1 and Mn-AP兲

hysteresis loops, as

presented in Fig.3

共

b

兲

. The Mn-AP hysteresis matches the Fe one up to 2 T, indicating the robustness of the magnetic coupling. At higher magnetic fields the observed partial re- duction in the Mn magnetic moment

共⬵40%兲

suggests a re- orientation of the Mn magnetization in a direction parallel to the field. Having identified the novel magnetic and electronic behavior of Mn-AP, we now address the distribution profile of Mn-AP across the interface, by means of XRMS results.

Figure 4

共

a

兲

reports the␪^-2␪ scan obtained at h␯^{= 640.4 eV} andT= 138 K, averaging the reflectivity curves for parallel and antiparallel relative orientation of photon helicity and magnetic field. Such a signal is insensitive to the magnetic moments of the sample and depends only upon the electron charge distribution and chemical properties. In Fig. 4

共

b

兲

we report the asymmetry ratio AR=

共r↑ ↑

−r↑↓兲/

共r

↑ ↑−r↑↓兲, where

共r↑↑兲

and

共r↑↓兲

are the reflectivity values measured for parallel and antiparallel orientation of the photon helicity and magnetization, respectively. The AR value gives infor- mation about the distribution of the Mn magnetic moment in the sample. We fitted the␪-2␪ scan using the thickness and roughness of the various layers

关Au,Fe,共GaMn兲As,GaAs兴

as fitting parameters. Absorption coefficients for the Mn species have been obtained experimentally from XAS measurements. The real part of the refractive index is obtained from the imaginary part, which is directly proportional to the absorption coefficient, through Kramers-Kroenig transforma- tion. The fitting results well reproduce the experimental curve

共

the values of the best fit parameters are listed in the caption of Fig. 4兲, in good agreement with TEM results.

Considering a distribution of Mn atoms antiparallely oriented with respect to Fe, we are able to reproduce maxima and minima of the measured oscillations: best agreement is found

1.2 0.8 0.4 XAS-PY(arb.units) 0.0

655 650 645 640

Photon energy (eV)

-5 0 5 10

XMCD(%)

(a)

Mn-1 Mn-2 Mn-AP Experiment Fitting result

20

10

0

-10

-20

Fe,Mn-1XMCD(%)

-4 -2 0 2 4

Magnetic Field (Tesla) -2 -1 0 1 2

Mn-APXMCD(%)

(b)

Mn-1 Mn-AP Fe (x0.5)

FIG. 3. 共Color online兲共a兲 Fitting results and experimental MnL_2,3 XAS/XMCD spectra 共black dashed curve and red/gray curve, respectively兲 using template spectra from Ref. 21 共Mn-1, blue/dark gray dashed curve and Mn-2 yellow/light gray dashed curve兲 and experimental Mn-AP XMCD 共green/light gray dot- dashed curve兲. Experimental curves correspond tot共Fe兲= 23 ML, T= 120 K, and H= 1 T. Best agreement between experimental XMCD and fit is obtained by using only Mn-1 and Mn-AP component.共b兲Hystereses cycles traced from the XMCDL₃peak height at the Mn and Fe edges, in percentage of the XAS sum peak 共I₊−I₋兲/2, forT= 120 K, Fe= 23 ML. Black arrows indicate the Fe XMCD scale 共left兲 and Mn XMCD one 共right兲. The Mn-1 共blue/

dark gray triangles兲and Mn-AP共green/light gray circles兲hystereses were extracted by fitting the L₃ XMCD spectra at each magnetic field, using the same fitting procedure of panel共a兲共linear combination of Mn-1 and Mn-AP components兲. The resulting Mn-1 behavior is purely paramagnetic, as expected above T_c of 共Ga,Mn兲As.

Experimental values of Fe XMCD 共red/gray diamonds兲 are multi- plied by 0.5 for sake of comparison.

10⁰ 10¹ 10² 10³ 10⁴

Reflectivity(Arb.Units)

40 35 30 25 20 15 10 5

Angle (degrees) 15x10^-3

10 5 0 -5 -10

NormalizedAsymmetry

(a)

(b)

Fit:

Exp:

Fit: A B

Exp:

Magnetization(Arb.Units)

-25 0 25

z (Å)

Au Fe

(c)

(d)

Ga1-xMnxAs Mn-APmagnetization profiles

A B

FIG. 4. 共Color online兲共a兲 Specular x-ray reflectivity at 138 K collected with a photon energy of 640.4 eV 共black circles兲 compared with fitting results obtained with the PPM program共red/gray line兲. Values used in the fitting are共thickness and roughness in Å, respectively兲共GaMn兲As, 369 and 3.0. Fe 27 and 4.1. Au 41 and 2.1.

GaAs substrate with roughness 1.0 Å.共b兲Experimental asymmetry ratio AR共blue/dark gray circles兲, as defined in the text. Comparison of the experimental AR with two simulated magnetic profiles, i.e., a 10-Å-thick magnetic layer of Mn ions located in the 共Ga,Mn兲As environment共red/gray curve, panel d兲or in the Fe film共green/light gray curve, panel d兲.共c兲Sketch of the experimental geometry used for XRMS experiments.

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using a 10-Å-thick layer of Mn atoms with a constant steplike magnetic profile in

共Ga,Mn兲As. We also simulated the

behavior of the same Mn thickness, but with Mn diffused into the Fe layer, in antiparallel configuration. The period of oscillations is almost equal but the positions of the maxima and minima are inverted. This excludes a sizeable presence of magnetic Mn atoms into the Fe film. We stress that the simple steplike magnetization profile used in the simulation, although able to reproduce the XRMS data, should be con- sidered as a lower estimate of the Mn-AP magnetic depth profile.

IV. CONCLUSIONS

In conclusion, by the use of a broad range of chemical sensitive spectroscopic tools we have provided a complete description of the Mn magnetic properties in fully epitaxial Fe/GaMnAs HS. The use of the magnetic proximity effect between Fe and Mn made it possible to reveal a novel electronic configuration of the interfacial Mn atoms, with different magnetic behavior with respect to the bulk ones. Our

original approach demonstrates that important insights in the fundamental interactions driving ferromagnetism in DMS- based systems can only be obtained by tailoring novel properties in controlled heterostructures. Future efforts in the direction of hybrid metal/DMS ferromagnetic structures may achieve the control of switching fields and Curie temperature, opening a completely new avenue for the design of hybrid metal/DMS/semiconductor structures for future spin- tronic devices.

ACKNOWLEDGMENTS

Financial support by the DFG through the SFB 689 is gratefully acknowledged. This work has been partially funded by CNR-INFM. Thanks are due to Alessandro Mirone for fruitful discussion and support for the use of the

PPMcode, and to Stefano Nannarone and Bruce A. Davidson for fruitful discussions on XRMS technique. A.V. was supported by the FVG Regional Project SPINOX funded by Legge Regionale 26/2005 and the decreto 2007/LAVFOR/

1461.

*panaccioneg@elettra.trieste.it

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